In telecommunication systems, the transmission channel often causes interference to data transmission. Interference occurs in all systems, but in particular in wireless telecommunication systems, the radio path attenuates and distorts the transmitted signal in a variety of ways. On the radio path, interference is typically caused by multipath propagation, various fades and reflections and also another signals transmitted on the same radio path.
In a radio communication environment, it is typical that the propagation conditions continuously change. Constant variation, or fading, occurs in a signal received both in a mobile station and a base station. However, the farther away transceivers are from each other, the more the transmitted signals attenuate.
Receivers of modern telecommunication systems must fulfil many requirements present in relevant standards. Generally, receivers are designed for a balance of sensitivity and linearity. In general, good linearity improves signal quality.
The sensitivity of the receiver depicts the receiver's ability to receive weak signals. A sensitive receiver can receive a signal further away from the transmitter (for example cellular system base station) than a receiver having poor sensitivity. Alternatively, the transmission power used by a base station can be smaller when a receiver with better sensitivity is used.
It is known that a large gain of a receiver will increase the sensitivity of the receiver. This applies especially to receivers utilising a low noise amplifier as a first stage (or as a one of the first stages) of the receiver. In such a receiver, the high gain amplifier amplifies the signal to such a high level that the noise coming from the following stages will be negligible. However, the high-level signals have the drawback that a lot of current (and power) is needed in the amplifying stage and the later stages to fulfil the linearity requirements.
In devices with limited battery capacity (such as mobile stations) it is not possible to use large amounts of power to compensate the effect of the large gain to linearity. Thus, sensitivity and the gain are limited to keep the linearity of the receiver on an acceptable level.
Another solution to increase the sensitivity of a receiver is to use two low noise amplifiers connected in series. However, this will cause high power consumption to these stages and the latter stages, because of the linearity specifications to be fulfilled.